stdenv, dep-licenses.sh: do not skip handling of other exit traps

See https://github.com/akinomyoga/ble.sh/issues/179

maintainers/scripts/dep-licenses.sh

···
 exitHandler() {
     exitCode=$?
     rm -rf "$tmp"
-    exit $exitCode
+    return $exitCode
 }
 
 trap "exitHandler" EXIT

pkgs/stdenv/generic/setup.sh

···
             echo "build failed with exit code $exitCode (ignored)"
             mkdir -p "$out/nix-support"
             printf "%s" "$exitCode" > "$out/nix-support/failed"
-            exit 0
+            return 0
         fi
 
     else
         runHook exitHook
     fi
 
-    exit "$exitCode"
+    return "$exitCode"
 }
 
 trap "exitHandler" EXIT